1. Field of the Invention
The present invention generally relates to image sensing systems and more particularly relates to an image sensor directly converting light intensity signals to digital signals without using A/D converters.
2. Description of the Related Art
There are many applications that need an imaging system to convert a target to an electronic format that can be subsequently analyzed, printed, distributed and archived. The electronic format is generally an image of the target. A typical example of the imaging system is a scanner and the target is a sheet of paper from a book or an article. Through the scanner, an electronic or digital image of the paper is generated.
An imaging system generally includes a sensing module that converts a target optically into an image. The key element in the sensing module that converts the target optically to the image is an image sensor comprising an array of photodetectors responsive to light impinged upon the image sensor. Each of the photodetectors produces an electronic (charge) signal representing the intensity of light reflected from the target. The electronic signals from all the photodetectors are readout and then digitized through an analog-to-digital converter to produce digital signals or image of the target.
It is known in the art that the electronic signals in the image sensor are serially readout, whereby the electronic signals may have passed a number of circuits that may affect adversely the quality of the electronic signals. For example, in CCD sensors, the electronic signals are serially shifted out from one charge storage to another charge storage. During the course of going through tens, perhaps, hundreds or thousands of the charge storages, the electronic signals may have been introduced to noise from other poor-performed charge storages or degraded and even distorted because of parasitic effects caused by parasitic capacitance, inductance and resistance of other components along the way. Similarly, these adverse effects exist in CMOS sensors as well. Therefore there is a great need for solutions leading to direct readout of the electronic signals.
U.S. Pat. No. 5,461,425 by Boyd Fowler and Abbas El Gamal discloses a CMOS image sensor with pixel level A/D conversion, which means that the electronic (analog) signals generated by the photodetectors are converted to a serial bit stream by an A/D converter connected at the output of each photodetector and formed in the immediate area of each photodetector within the sensor. Thus, a separate digital stream for each photodetector (pixel element) is output from the sensor and the parasitic effects and distortion are minimized.
Attaching an independent A/D converter to each photodetector may be an expensive approach. If an image sensor, is of high resolution, for example, a linear array of 300 dpi for 9 inch width or 1000 by 1000 area array would require 2,700 A/D converters in the linear array and one million A/D converters in the area array. It is known in the art that the realization of a large quantity of 8-bit or 12-bit A/D converters can occupy a fairly large area in the array, which inherently means the higher cost of the sensor. Hence it is desirable for an image sensor having the same features but without using the individual A/D converters for all the photodetectors in the image sensor.
The present invention has been made in consideration of the above described problems and needs and has particular applications to image sensors used in scanners, digital cameras and computer vision systems.
According to one aspect of the present invention, an image sensor can be fabricated as Complementary Metal-Oxide Semiconductor (CMOS) or Charged Couple Device (CCD) device in a format of either one-dimensional array or two-dimensional array. The image sensor comprises a plurality of photodetectors, each responsive to light impinged thereupon and producing an electronic leakage current or charge signal in light integration. Each of the photodetector is connected to a time measurement module that produces a digital representation of the charge signal. The digital representations of all the charge signals from the photodetectors are then sequentially read out as digital signals from a sequence of register circuits, each coupled to one of the time measurement modules.
The time measurement modules and register circuits are digital circuits and therefore can be fabricated together in the image sensor without significantly increasing the size and cost of the image sensor. Image sensors employing this invention produce signals that are not only in digital format but also of high fidelity.
One of the distinctions of the present invention from prior art system is that the image sensor can produce digital signals directly without using A/D converters. One of the benefits and advantages of the present invention is the signal fidelity. In prior art systems, the charge signals have to go through tens, perhaps hundreds or thousands of circuits, suffering various parasitic effects that cause the charge signals distorted. Using the present invention, the charge signals are converted to a digital format right after the photodetectors, minimizing possibilities of being distorted.
According to one embodiment, the present invention can be implemented as a system for generating digital signals in a sensor, the system comprising:
an array of photodetectors, each responsive to light impinged thereupon and independently producing a charge signal after the photodetectors are collectively reset by a reset signal;
a counter receiving a time mark signal and producing a count number with reference to the time mark signal;
a plurality of time mark measurement modules collectively receiving a reference signal and the count number from the counter, each of the time mark measurement modules coupled to one of the photodetectors and outputting a digital representation of the charge signal from the one of the photodetectors with reference to the reference signal and the count number from the counter, and
a plurality of register circuits, each connected to one of the time mark measurement modules and receiving the digital representation therefrom, wherein the digital representation from each of the time mark measurement modules is sequentially shifted out to form the digital signals.
According to another embodiment, the present invention can be implemented as a method for generating digital signals in a sensor, the method comprising:
accumulating incident photons in a photodetector; the accumulated incident photons causing a charge signal;
comparing the charge signal with a reference signal having a level using a comparing circuit;
producing a pulse signal by the comparing circuit when the charge signal reaches the level of the reference signal;
measuring time elapsed by a latch circuit for the charge signal to have reached the level of the reference signal; a measured result thus obtained when the latch circuit receives the pulse signal; and
dumping the measured result in a register circuit for output.
Accordingly, an important object of the present invention is to provide a generic solution for direct readout of the charge signals from the photodetectors in an image sensor to minimize possible signal distortions.
Other objects, together with the foregoing are attained in the exercise of the invention in the following description and resulting in the embodiment illustrated in the accompanying drawings.